Microwave heating



Sept. 30, 1952 F. A. GROSS 2,612,596

MICROWAVE HEATING Filed Feb. 18, 1947 AVVEA TOR FRITZ A. 65 055 PatentedSept. 30', 1952 U I STATES PATENT-1 O FICE f H 1,612,596 p l mcaowAvEunarmea Fritz A. Gross, Weston, Mass'. ,iassignor to RaytheonManufacturing Company, Newton, Mass acorporation ofDelaware' lApplication February18, 1947, Serial No'. 729,237

6 Claims; (c1. ai s- 11) This invention relates to high-frequencyapparatus, and more particularly to a means for selectively heatingdielectric masses.

An object of this invention is to devise a. means for. enablingsubstantially uniform heating of nonuniform dielectric bodies to beaccomplished by radio-frequency energy.

Another object is to provide a means for causing selective coupling ofradio-frequency energy into selected portions of food bodies.

A further object is to provide a means for producing selective couplingof radio-frequency energy into the central portions of frozen foodbodies.

The foregoing and other objects of the invention will be best understoodfrom the following description of some exemplifications thereof.reference being had to the accompanying drawing, wherein:

Fig. 1 is a vertical section through an oven utilizing my invention; I

.Fig. 2 is a partial vertical section through an oven utilizing myinvention, shown on an enlarged scale for purposes of explanation;

Fig. 3 is a partial vertical section of an oven utilizing a modifiedformof my invention; and I Fig. 4 is a section taken along line 4-4 ofFig. 3. v

It has been found that masses of frozen food, for example massescomposed of slices or layers of meat, as received from the foodprocessors, are

generally arranged in such a manner that the slices or layers of frozenfood overlie each other in the central region or regions of themass,-and do not so overlie each other in the outer region or regionsthereof. Such a mass will have nonuniform radio-frequency energy losscharacteristics because of the nonuniformity in thickness of thedielectric material.

I have found that, when such a mass of frozen food is attempted to beheated by the use of radio-frequency energy, the ice crystals naturallypresent in all frozen foods tend to reflect the radio-frequency energyand, since there is an increased thickness of food in the central regionof the ,mass and therefore also an increased though vI have discussedtheproblem. only with respectto frozen food bodies or masses composed ofmeat, it will be'appreciated that a similar problem arises whenever itis desired to heat in a substantially uniform manner, by radio-frequencyor microwave energy, other kinds of food, or in fact any dielectric bodywhich has nonuniform radiofrequency energy loss characteristics. In allsuch cases it is desirable to be able to produce selective coupling ofthe radio-frequency energy into selected portions of the dielectricbodies in order to heat them uniformly.

Now referring to Fig. l, a hollow thin-walled metallic enclosure orcavity l preferably ha the shape of a rectangular parallelepiped, thisenclosure serving as the ovenoij aradio-frequency or microwave heatingdevice. Radio-frequency energy is supplied ,to'the interior of cavity Ifrom aperture being located somewhat above the horizontal mid-plane ofthe cavity. An access aperture 4 is provided in the front wall of-ovenI, this aperture being closable by means of a hinged metal door 5. Whendoor 5 is in its closed position, it will be seen that thecavity I isentirely closed, except for the energy supply aperture 3 abovedescribed,through which radio-frequency energy in the microwave region of thefrequency spectrum, that is, wave energy having a wavelength of theorder, of thirty centimeters or less. is supplied to ,said activity. Inthe specification and claims. it is to be understood that the termwavelength" is :used to refer to the predetermined wavelength oftheradio-frequency energy suppliedto the cavity, 7 1 t On the irmer, faceof the bottom metallic. wall 6 of the oven is placed a dielectric blockor member I, which, for example, may be prismoidal or cylindrical andwhich has a thickness or height a. Block I may bemade of a lowradio-frequency loss, synthetic resin, such as'polystyrene, for example.On the upper surface of block 1 is placed a dish 8 which contains thedielectric body or mass 9 to be heated by radio-frequency energy.

In this figure the dielectric body 9 is represented as a plurality ,oflayers of frozen food, the thicknesses of the individual layers beingexaggerated for purposes of clarity. As shownin this figure,

th layersoverlie each other or, overlap in the central portion; orregion 01., theme-5s, thereby producing arr-increased thickness of .thebody in this region. Block I is of such size that it underunderlain bysuch block; in other words, the coupling to such portion will beincreased or made more effective than if such block were not used.

I do not know exactly how orwhy-this result is accomplished, but mypresent-theory of operation of such a block is set out hereinafter.

When radio-frequency waves are supplied". to

the interior of cavity I, reflections of such waves 4 so that thedistance between points A and C is approximately a quarter-wavelength.Since this is the case, point D (the point of voltage maximum) islocated within the confines of body 9, even though the thickness of saidbody is appreciably less than a quarter-wavelength. It will be seentherefore, that, by the addition of block 1,

the body il -has in effect been; moved to the region of voltage maximumof the standing wave system. The heating effect of radio-frequencyenergy in -a'dielectric body is increased in regions of standingqwavevoltage maxima, so that there is an increased heating effect produced inbody 9 by I theaddition of block 1. In other words, theradiofrel'qiiericy energy coupled 'iiitdthe' body 9 in regions underlainby is more effectively 3 block 1, and such regions can be effectivelyselected by selecting the area and position of block from the metallicwalls, and particularly from" the bottom wall, of said cavity cause asystem of standing waves to be established therein, a voltage minimum insuch systemtending ,to' begestablished at bottom wall 6' because of lthe me tallic short-circuiting effect of sucha wall..

Now referring to Fig. 2, whichushowsin a diagrammatic manner the lowerend'of the oven, the dielectric body!) to be heated is shownfas resteing directly on the uppersurface of dielectric block 1, withouttheinterpositionof a: dish or plate. Such a showing is merelyforpurposes of ease in explanation, however, since a dish isordinarily usedbetween the block and the body;

First we will consider the' situation existing when a block] is notprovidedbetween-ttie body 9 and the bottom'wall or 'iloonof the Seven;in which case-the body Swill be restingwith itsslow er face I lldirectly on the metallic oven floor; IBecause ofthe short-circuitingeffect of this metallic surface, a standing wave voltage minimumjoi'node would tend to be establishedzat point A on this surface, asexplained-above Point B, which is spaced a distance of a quarterwavelength'from point A, would therefore be ata voltage' maximum. Sincethe thicknessof bodys isitappreciably less than'a quarter=wavelength,point B is located in thespace above body 9.

Moreover, there is a very substantial impedance mismatch between body 9'and the airinside cavity 1. Thus, when body QiS re'sting 'dir'ectly'onthe bottom-wall 6 f-the'cavity or"oven, 'a very substantial proportionofthe microwave energy which impinges onthe upper surface of-"body"9 isreflected, resulting in ineifecti-ve heatirigof the interior portionsand alsoof the lowen surface lll of said bodyj-sinceunderthesecai'iditions there is no opportunity for-energy-reflected from theoven bottom wall 6 and/or the lower portions of the oven side walls toimpinge on: the lower surface In of body 9,. said-lowerisurfacerestingdirectly on bottom wall .6.

Now we will consider the situation; present when a block. 1 isprovided.betweenbody.9 and bottom wall 6 on the oven. {B100}? I isy madeof a material whichhas a dielectric constant, such that an: effectiveimpedance match. is iprovided between the body 9 and the block'li asaresult, there is no appreciable reilecti'on of wave energy at the 1 -9interface. 'lnflth sjtanding' wave system 'a voltage agaimte'ndsto' beestablished at the metallic wallt, in the plane of which wallpoin'tC nowlies, and a voltage maximum at p'oint'D, which is'a quarter-wavelehgthaway 'from point C. The dimension-ordistance a of Fig.- l, which is thethickness or height of block'l, is approximately a' quar'ter avelngth,

1. Since block I is made of a material having a low radio-frequencyloss, said block is not heated appreciably by theradio-frequencylenergy.

' Although the above effect of moving body 9 to the region of voltagemaximum'of the'standing wave system may help to'explainthe increasedselecti'veheating effect made possible by the use of block 1, it ispossible and perhaps preferable to explain said effect in another way.As previously described, block 1 provides an impedance match to body 9,so that there is no appreciable reflection of wave energy attheT--9"interface. Block 1 has a height suflicie'nt to allow energyreflected from the bottom wall 6 and from the lower portions of the ovenside walls to impinge on the lower face ID of body 9, at points both outside the area of block 1 (such as points E and F) and inside the area ofblock 1"(s'uch as point A), block I efficiently conducting such energy,

At pointson the bottom face ill of body 9 outside the area of block I,such as E and F, there is a meat-air interface which providesa'discontinuity or impedance mismatch at all such points. Therefore,there is a substantial reflection of energy impinging on face ID at eachand every such point} so that the addition of block I does not-increasethe coupling of energy into body 9 atsu'ch outside points. 7

However, at pointson the bottom face [0 of body 9 inside the area ofblock 1, such as A, there is a l+9-interfa ce or impedance match, sothat no appreciable reflection of energy impinging on face l0throughblock'l'l' occurs at these points. Therefore; the addition-of block 1does substantially increase the coupling of the radio-frequency energyinto body 9 at such inside points, or into those areasof said saidblockv v =Referring 'againto Fig. 1, block 1 is positioned under-the centralregion of body 9. where the layers of food overlap, so that increasedheating of the body 9'; by radio-frequency'energy is effectuatedin thisregion. which is where it is desiredi- Newieremng to Figs'hB and 4,these figures disbody which are underlain by close" an oven I in.whichsele'ctive"heatingof a the dielectric blocks, in order to heatother standardized aggregations of foods. The lower face of plate I lrests on the bottom wall 6 of the oven l, and the unit |l-I4 isremovably inserted into said oven through aperture 4.

A,sectionalized dish 15 has a plurality of food masses or bodies 16, H,and I8 therein, one mass being in each section of said dish. Dish 15with itscontents is placed in oven 1, the bottom face ofsaid dishresting onand being supported by theupper faces of blocks l2l4. Blocksl2l4 are so located that each block underlies a separate correspondingone of the food bodies Iii-l8, and the blocks are of such a size thateach block underlies only the central portion of the corresponding foodbody. For example, block l2 underlies the'central portion of body I6,block l3 underlies the central portion of body l1, and

block 14 underlies the central portion of body l8.

Each of blocks l2, l3, and M has a height of approximately aquarter-wavelength and a dielectric constant such that it will act as animpedance-matching device, as explained above in.

connection with block 1. Therefore, when radiofrequency energy issupplied to cavity I, said energy will be selectively coupled into theportion of each food body which is underlain by the corresponding block,resulting in a selective heating of each of the food bodies l6- l8, inwhich the central portion of each food body is heated more eiiicientlythan the outer portions thereof, because the central portion of each ofthe bodies is underlain by a suitable block.

It will be seen, from all of the above, that I have devised a means forselective radio-frequency heating of dielectric masses, and that, byproper positioning of such means, substantially uniform heating of anynonuniform dielectric body by radio-frequency energy may be easilyaccomplished.

Of course, it is to be understood that this invention is not limited tothe particular details as described above, as many equivalents willsuggest themselves to those skilled in the art. For example, the singledielectric block 1 of Fig. 1 may be mounted on a thin metallic plate,similar to plate ll of Fig. 3, if desired. Various other variations willsuggest themselves. It is accordingly desired that the appended claimsbe given a broad interpretation commensurate with the scope .of thisinvention within the art.

What is claimed is:

1. In combination, a metallic enclosure, means for. radiatingradio-frequency energy into the interior of said enclosure, a dielectricbody in said enclosure substantially spaced from the walls thereof, anda coupler for causing differential coupling of said energy to said bodyindifferent portions thereof, said coupler being positioned between oneenclosure wall and said body.

2. In combination, a metallic enclosure, means forradiatingradio-frequency energy into the interior of said enclosure, dielectricbody in said enclosure substantially spaced from the walls thereof, andmeans for causing increased coupling ofsaid energy to said body inselected portions thereof, said last-named means being positionedbetween one enclosure wall and said selected portions of said body.

3. In combination, a metallic enclosure, means for radiating microwaveenergy of a predetermined wavelength into the interior of saidenclosure, a dielectric body in said enclosure substantially spaced fromthe walls thereof, and a dielectric member, substantially one-fourth ofsaid wavelength thick, positioned between one enclosure wall and saidbody, a portion only of the surface of said body being positionedclosely adjacent said member.

4. In combination, a metallic enclosure, means for radiating microwaveenergy into the interior of said enclosure, a dielectric body in said enclosure substantially spaced from the walls thereof, and a dielectricmember positioned between one enclosure wall and said body and incontact with the latter, a portion only of the surface of said bodybeing positioned closely adjacent said member, said member being made ofa material having a wave impedance such that there is no appreciablereflection of wave energy at the interface between said member and saidbody.

5. In combination, a metallic enclosure, means for radiating microwaveenergy of a predetermined wavelength into the interior of saidenclosure, a low-loss dielectric member, substantially one-fourth ofsaid wavelength thick, in said enclosure and supported by the bottomwall thereof, and a dielectric body to be heated supported by saidmember, a portion only of the surface of said body being positionedvclosely adjacent said member.

6. Incombination, a metallic enclosure, means for radiatingradio-frequency energy into the interior of said enclosure, 2. low-lossdielectric member in said enclosure and supported by the bottom wallthereof, and a dielectric body to be heated supported by said memberspaced from the walls of said enclosure, said member underlying only aselected portion of said body and being made REFERENCES CITED Thefollowing references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,259,318 Mouromtsefi Oct. 14,1941 2,280,824 Hansen et a1 Apr. 28, 1942 2,370,161 Hansen Feb. 27, 19452,370,624 Gillespie Mar. 6, 1945 2,396,004 Gilbert Mar. 5, 19462,407,690 Southworth Sept. 17, 1946 2,433,368 Johnson et al Dec. 30,1947 2,464,403 Klingaman Mar. 15, 1949 2,464,404 Gillespie Mar. 15, 1949f 2,465,102 Joy Mar. 22, 1949 2,495,415 Marshall Jan. 24, 1950 2,497,670Hanson et a1. Feb. 14, 1950 2,500,752 Hanson et a1 Mar. 14, 19502,504,109 Dakin et al Apr. 18. 1950 2,567,748 White Sept. 11, 1951 OTHERREFERENCES Hutcheson: Electronic Torch, The Welding Engineer, December1945, page 90.

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